This application relates to and incorporates herein by reference Japanese patent application No. 2001-220174, which was filed on Jul. 19, 2001.
The invention relates to a thin-film type flow sensor having a thin-film part in which patterned resistor films are sandwiched between a pair of insulator films.
In general, thin-film type flow sensors have a thin-film part in which a lower insulator film, resistor films made of metal, highly-doped semiconductors, or the like, and an upper insulator film are formed on a substrate in succession to make a lamination. The resistor films consist of a heater and a thermometer (a fluid thermometer and a temperature detector) which are patterned in a wiring configuration.
The proposed examples include one described in Japanese unexamined patent (JP-A) publication No. 2000-146656, in which the temperature of the heater is controlled to a predetermined temperature, which is higher than that detected by the fluid thermometer. The flow rate of fluid is detected from temperature variations of the temperature detector caused by the flow of the fluid.
Such flow sensors are typically mass-produced by the following steps: forming a lower insulator film and a resistor film in succession on a semiconductor wafer; patterning the resistor film by photolithography-based etching; forming an upper insulator film; and then cutting the wafer into chips or substrates.
In the patterning of the resistor film, however, the etching causes variations in the wiring width over the wafer surface. The variations produce the problem that resistance ratios between individual resistor films (between the heater and the thermometer films, between heater films, or between thermometer films) of one chip, or sensor, (hereinafter, referred to as in-chip resistance ratios) vary greatly from those of another chip (the ratios vary from one sensor to another).
The flow sensors can detect the flow rate of fluid, for example, with bridge circuitry or the like formed with the individual resistor films. The resistance ratios between the resistor films thus have a large impact on the sensitivities of the sensors. Consequently, when the in-chip resistance ratios of the mass-produced chips vary greatly, output correction and other processing of external control circuits may be complicated.
The same problems also occur when ratios of TCR (thermal coefficient of resistance) among the resistor films of a single chip vary greatly from one chip to the next.
The inventors have studied the causes of these variations in the in-chip resistance ratio and TCR ratio. The resistances are determined by the thicknesses and wiring widths of the resistor films. The values of TCR also depend on the thicknesses and wiring widths. The thickness, in turn, depends on the film forming apparatus. Although the thickness varies greatly over a wafer surface, e.g., between the wafer center and periphery, the thickness varies only slightly within each chip; thus the variation hardly affects the in-chip resistance ratios and TCR ratios.
The wiring width depends on variations in the etching of the resistor film. The variations of the wiring width are attributed to the following factors: 1) The differences between the line widths of the mask used in the photolithography and the widths of the lines actually etched (hereinafter, referred to as etching variations) vary over a wafer surface; and 2) The magnitudes of the etching variations and the contributions of the etching variations to the resistances and TCRs differ between resistor films of greater line width and those of smaller line width.
Conventional flow sensors typically have resistor films as shown in FIGS. 6A and 6B. That is, a heater film 51 is given as great a wiring width as possible in order to generate heat at lower voltages. Meanwhile, thermometers 3 and 4 are given as small a wiring width as possible since higher voltages are to be obtained under smaller currents for the sake of less heat generation. Hence, the variations in wiring width caused by etching have a greater impact on the thermometer films 3, 4, which have a narrower wiring width, than on the heater film 5xe2x80x2, which has a greater wiring width.
Consequently, as far as factor 2) is concerned, resistor films of narrower wiring widths are more susceptible to the etching variations mentioned above. In addition, the values of the etching variations differ largely between heaters and thermometers. Thus, the in-chip resistance ratios vary greatly from one chip to the next.
For example, in FIGS. 6A and 6B, suppose that the heater film 5xe2x80x2 has a wiring width W1 of 20 xcexcm and the thermometer films 3, 4 have a wiring width W2 of 3 xcexcm. The etching variations near the wafer center shall be 0.1 xcexcm for the heater film 5xe2x80x2 and 0.3 xcexcm for the thermometer films 3, 4. On the wafer periphery, the etching variations shall be 0.15 xcexcm for the heater film 5xe2x80x2 and 0.45 xcexcm for the thermometer films 3, 4.
As for the in-chip resistance ratios, the ratio between the resistance of the heater film 5xe2x80x2 and that of the thermometer films 3, 4 ((heater resistance)/(thermometer resistance)) is considered. A chip cut from a location near the wafer center has an in-chip resistance ratio x1 of ((20xe2x88x920.1)/(3xe2x88x920.3)), and a chip cut out of the wafer periphery an in-chip resistance ratio x2 of ((20xe2x88x920.15)/(3xe2x88x920.45)).
The ratio between the two resistance ratios x1 and x2, or x1/x2, is 0.947. As seen above, the narrow and wide wiring widths differ in the degree of contribution of the etching variation to the resistance ratios near the wafer center and on the wafer periphery. The in-chip resistance ratio thus varies greatly over the wafer surface, i.e., from one chip to another. The same is true for the TCR ratios. The reason is that TCR depends on line widths (the greater the line width, the greater the TCR).
In view of the foregoing problems, it is an object of the present invention to provide a flow sensor that includes a lower insulator film, resistor films, which include a patterned heater and thermometer, and an upper insulator film laminated in succession on a substrate cut from a wafer, wherein the variation from one chip to another of in-chip resistance ratios and in-chip TCR ratios are minimized.
As mentioned above, narrow sections and wide sections of wiring differ in the degree of etching variations between locations near the center and locations near the periphery of the wafer. The etching variations depend on the line width and line spacing of the resistor films. When all the resistor films have the same line width and line spacing, the in-chip resistance ratios are supposed to be invariant between chips, even if the values of the etching variations differ over the wafer surface.
Thus, in the foregoing example described in conjunction with FIGS. 6A and 6B, suppose the line width of the heater film 5xe2x80x2 and the line widths of the thermometer films 3, 4, or W1 and W2, are both set at 3 xcexcm, for instance. The ratio x1/x2 is expressed as {(3xe2x88x920.3)/(3xe2x88x920.3)}/{(3xe2x88x920.45)/(3xe2x88x920.45)}=1. In this case, the disparity in the values of etching variations within the wafer surface is cancelled to make the in-chip resistance ratios invariant from one chip to the next, or from one sensor to another.
In addition to the findings from the study of the inventors, the present invention has been achieved with consideration also given to the fact that the above-mentioned problems occur because both wide and narrow sections of the resistor films in earlier, conventional flow sensors are formed by a single, contiguous line, or strip.
That is, according to a first aspect of the present invention, a flow sensor includes a lower insulator film, resistor films, and an upper insulator film laminated on a substrate such that the resistor films are located between the insulator films. The resistor films include a patterned heater and thermometer. At least a section of one of the resistor films has a wiring configuration in which resistor elements are connected in a parallel manner.
The resistor films of earlier devices have contained sections of different wiring widths; that is, wide and narrow. In contrast, according to the present invention, the resistor films that previously were relatively wide have been configured as a plurality of resistor elements connected in a parallel manner. The wiring width of the individual resistor elements can thus be made the same as that of the relatively narrow sections. In short, all the resistor films will have identical wiring widths.
Thus, the disparity in the etching variations over the wafer surface, which have been due to differences in wiring widths, can be minimized by adopting the configuration of the present invention. According to the present invention, the variation from one chip to the next of resistance ratios and TCR ratios can be minimized.
In a second aspect of the present invention, at least part of the heater, which is one of the resistor films, has a wiring configuration in which a plurality of resistor elements are connected in a parallel manner.
As stated previously, the heater resistor film typically is the part that requires a greater wiring width than the other resistor films, such as the thermometer. According to the present invention, at least part of the heater has a wiring configuration such that resistor elements of a plurality of resistor elements are connected in a parallel manner. The wiring widths of the individual resistor films will thus be identical, which achieves the effect of the first aspect of the invention.
In a third aspect of the invention, at least part of the thermometer, which is included in the resistor films, has a wiring configuration in which a plurality of resistor elements are connected in a parallel manner.
In earlier devices, the wiring width of the thermometer was increased when there was a need to reduce the resistance of the thermometer. In some cases, the wiring width of the thermometer was even greater than that of the heater. The present invention is suitable for such cases. At least part of the thermometer is has a wiring configuration in which a plurality of resistor elements are connected in a parallel manner, thus the wiring widths of the individual resistor films are essentially identical. This achieves the effect of the first aspect of the invention.
In a fourth aspect of the invention, at least one of the heater and the thermometer has a wiring configuration such that resistor elements are connected in a parallel manner. Here, the heater and the thermometer may both have a section that includes parallel resistor elements of the same wiring width.
As a result, at least part of the heater and part the thermometer each have a wiring configuration in which a plurality of resistor elements are connected in a parallel manner. The heater and the thermometer will both have same wiring width. Consequently, according to the present invention, variation from one chip to the next of the resistance ratios and TCR ratios of the resistor films of the same chip can be minimized.